Emily Dickinson Lexicon

AB'A-CUS, n. [L. abacus, any thing flat, as a cupboard, a bench, a slate, a table or board for games; Gr. αβαξ. Usually deduced from the Oriental, אבק abak, dust, because the ancients used tables covered with dust for making figures and diagrams.]

1. Among the Romans, a cupboard or buffet.
2. An instrument to facilitate operations in arithmetic; on this are drawn lines; a counter on the lowest line, is one; on the next, ten; on the third, a hundred, &c. On the spaces, counters denote half the number of the line above. Other schemes are called by the same name. The name is also given to a table of numbers cast up, as, an abacus of addition; and by analogy, to the art of numbering, as in Knighton's Chronicon. – Encyc.
3. In architecture, a table constituting the upper member or crowning of a column and its capital. It is usually square, but sometimes its sides are arched inwards. The name is also given to a concave molding on the capital of the Tuscan pedestal; and to the plinth above the boultin in the Tuscan and Doric orders. – Encyc.